Kell Brook looks natural at middleweight, ready for Gennady Golovkin

When middleweight champion Gennady Golovkin's next fight was announced, shock was the reaction.

When Kell Brook, the welterweight champion moving up two weight classes for the challenge, showed off his new body, shock was the reaction once more.

The undefeated Brit actually outweighed GGG at both WBC weight checks (30 and 7 days out). And Brook wasn't fleshy like Amir Khan, his countryman who moved up in weight and was knocked out by Canelo Alvarez in May.

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Brook (36-0, 25 KOs) is lean with muscle definition, but also appears strong. Of course, no one knows how his chin will hold up against boxing's most fearsome puncher.

But if appearances are any indication, Golovkin, he of the 22 consecutive knockouts, will have a fight on his hands Saturday at the O2 Arena in London (5:30 p.m. ET, HBO).

"I've always known that I'm a big guy. I've got big legs; big arms as you've seen at the last weigh-in," Brook told Paste BN Sports during a phone interview. "I'm not fleshy; I'm in great shape. Once we get the water out (of my muscles), I'm going to look even better definition-wise."

Of course, boxing has never been a bodybuilding contest. Golovkin, a 34-year-old Kazakh who now calls Los Angeles home, is the sport's most-avoided fighter for a reason. When he lands flush, opponents react in ways you only witness when a devastating puncher is inflicting the punishment.

Guys like Tommy Hearns. Mike Tyson. George Foreman. Bob Foster.

It's not just the power. GGG, an Olympic silver medalist, owns an impressive boxing IQ, has an educated jab and is one of the best at cutting off the ring. All these skills have combined to make him BoxingJunkie's No. 2 pound-for-pound fighter.

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But for all the accolades GGG has collected; all the belts he's racked up; all the time's he had his hand raised, he hasn't faced an opponent nearly as talented as Brook.

That's no fault of Golovkin's, who has tried his darnedest to secure a mega bout with Canelo. But even if Brook is moving up in weight, he's levels ahead of any fighter GGG has topped.

"Everybody he's fought, I would beat and vice versa," Brook, 30, said. "This is a test. He's fighting an unbeaten fighter. I have the pedigree and style (to beat him)."

Part of the reason he moved up in weight is because like Golovkin, he couldn't entice the top guys in his weight class to fight him. Brook knows he was accused of picking and choosing opponents, but says the GGG challenge just proves that he always wanted to fight the best.

If Brook carries his speed and power 13 pounds north, GGG could be in for a big challenge. Golovkin (35-0, 32 KOs) is only one inch taller and his reach is just two inches longer. In fact, "Special K" feels he could have been fighting at 154 pounds all along.

"Just self-belief. My speed, my power, my boxing, all rolled into one," said Brook, who defended the 147-pound title he won from Shawn Porter three times. "I need everything I've got to beat Triple G.

"He is a solid warrior. It's going to be a very tough fight. I've put myself in the position and I'm going out there to upset the apple cart."

It's a minor miracle that Brook is even fighting at the top level right now. He was stabbed in the left thigh with a machete during a trip to Spain in 2014, and required 32 metal staples to close the wound.

"(The doctors) said 'might be a problem for you boxing again,' but they gave me a light at the end of tunnel," he recalled. "I got through that chapter in my life and it made me a stronger fighter; more mentally strong. It's kill or be killed in this game. I went on to defend my titles."

Brook has been sparring with countryman Liam Smith, who fights Canleo next week, to prepare for the weight jump. He said it's been nice to feel the power, and he's more than held his own.

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But Brook isn't delusional - he knows he's a big underdog for a reason, but he carries full belief he has what it takes. If nothing else, he wants to prove it to himself.

"I think he's going to be in shock when he sees the shape I'm in and the size I'm at and he comes out and feels the speed and power from me," Brook boasted. "He's going to know he's in for one helluva fight.

" … Everyone writes me off and says I have no chance. That gets me excited and makes me run every morning. I live for this sport. This is the moment. This is it for me. This is the biggest moment of my career.

"I think a lot of fighters have already lost before they get in the ring (with GGG). They hear all the things about his power and he walks through walls and breathes fire. For me, it's different. I've looked death in the face. I'm going to do what I want to do (on Saturday)."

(Gennady Golovkin and Kell Brook pose at a press conference ahead of their fight in London. Credit: Bryn Lennon, Getty Images )